Ultrasound pulse-echo apparatus is known and available for non-invasive imaging of humans and animals in studying the internal anatomy, as well as for nondestructive testing of engineering materials for flaws, inclusions, cracks or fissures. In such systems, a series of very short ultrasound pulses are transmitted through a suitable conducting medium (usually water) to the object under test. The returning echoes from increasing depths in the object arrive at the receiver at successively increasing time delays with respect to the time of pulse initiation. These echoes are displayed on a cathode-ray tube (CRT) in an A-, B-, or C-scan presentation as the ultrasound beam is scanned over the object to produce a television-type image of the interior of the object. The amplitude or strength of the echoes is displayed as a corresponding brightness level of the image; viz, a strongly reflecting internal structure, such as hardened artery walls, appear brighter on the display than more weakly reflecting structures. This relative brightness or gray scale in the display serves as a useful tool in the diagnosis of the flaw or diseased organ, as the case may be.
In such ultrasonic imaging systems, as the object is being interrogated by the unltrasound beam it produces a set of ultrasound echoes which represent in real-time the various acoustical interfaces within the object. The amplitude of such echoes is proportional to both the intensity of the transmitted ultrasound pulse and the mechanical impedance discontinuity at the interface, while the time-spacing between the echoes is proportional to the physical spacing of the respective reflecting interfaces. In general, the object medium absorbs ultrasound energy (which is converted within the object to heat) and consequently the echoes from increasing depth in the object are imaged by gradually diminishing incident ultrasound energy, resulting in decreasing echo strengths from deeper lying structures. It is known in the prior art to offset the above-described effects due to absorption in the object medium by increasing, monotonically with time, the electronic gain of the signal processing circuit which couples the electrical signals from the transducer to the display. This is known as time-again compensation (TGC) and is arranged to provide successively higher gain to echoes arriving later in time; that is, from increasing depths in the object in an attempt to compensate for the absorption effects.
It has been found, however, that mechanisms other than absorption effect exist which attenuate the ultrasound pulse as it progresses deeper within the object The present invention is based on the recognition of these additional effects and how they can be compensated to improve the performance of ultrasound apparatus.
The ultrasound absorption properties of object media are generally given by EQU I = I.sub.o e.sup.-.alpha.z = I.sub.o e.sup.-(.alpha./c)t Eq. ( 1)
where I is the intensity of the ultrasound pulse, z is the propagation path-length, .alpha. and c are the attenuation coefficient and ultrasound velocity in the medium, respectively, and t is the arrival time at the transducer of the reflected pulse. Thus, the correction for object medium absorption effects is simply a time-gain compensation of the form e.sup.+(.alpha./c). In addition to absorption there are various other attenuation mechanisms, particularly the geometrical damping mechanisms associated with beam spreading and/or focussing, and reduction in the incident ultrasound intensity at successive interfaces due to reflection and scatter at the preceding interfaces and/or media. In an attempt to take these other effects into account, TGC circuits have heretofore been modified, as by operator-controlled adjustment of certain parameters of the TGC circuit, to provide arbitrarily-shaped timegain characteristics. The extreme flexibility introduced by the operator control of the TGC circuit, however, not only subjects the equipment operator to a baffling array of knobs and controls, but gives rise to the much more fundamental problem of lack of repeatability of the image insofar as the operator cannot reproduce the complex time-gain function with any exactness at a later point in time.
It is a primary object of the present invention to provide an improved ultrasonic pulse-echo apparatus capable of providing compensation for all three of the major attenuation effects outlined above with a view toward simplification of the apparatus from the operator's point of view, and improvement in echo detection and repeatability of the diagnostic results over that obtainable with pulse-echo apparatus heretofore available.